I am about to install the thru hull for my exhaust, but I am not sure of the ideal height to install it. THe engine is mounted with about 1/3 below the water line. The manifold is well above the waterline. Should I place the thru hull above the height of the exhaust, level to it, or below it? As this is a multihull, the exhaust will not be submerged in any conditions unless it is well below the manifold.

A few inches above the water line, pointing down, is triditional. If it's too high it spits on docks, people, other boats & dinghy's. Also you do want it way below the riser. If it were to get hit by a wave, make it travel up hill........................._/)

Thanks everyone. Great diagrams GORD. Because of the set up I have on the tri, the ideal installation is to exit to the side instead of the transome. I was going to go with a water lock, but if I can save the money, that would not hurt my feelings at all. The run will be about 6', and dropping 1/2" per foot will put me at about 6" above the waterline, so those specs are easy to meet. With my design, the exhaust will exit under the connecter, so no exhaust will rise to the deck. I also have no worries about the exhaust spraying onto docks or other boats for the same reason.
I have seen many fishing boats with the exhaust exiting to the side, but with the exception of double enders, I have not seen this on a sailing vessel. That in mind, I consider this a nonconventional installation, so I am open to any suggestions.

Cool! I will be taking some time off next week to work on the tri, so hopefully I will have some updated photos soon. (If it ever stops raining)
Just to clarify, if the whole system is above the waterline, should I still install a water lock?

Kai Nui once whispered in the wind:Cool! I will be taking some time off next week to work on the tri, so hopefully I will have some updated photos soon. (If it ever stops raining)
Just to clarify, if the whole system is above the waterline, should I still install a water lock?

IT can't hurt. Having a waterlock with mute some of the exhaust noise and might help if you took some water up the pipe.